Simulated Gaia DR5 astrometry for an 8 M_J planet on a 4 au orbit around a 0.8 M_⊙ star located 100 pc away (G = 11). The two panels in the upper right corner show the R.A. (α) and decl. (δ) measurements and the individual along-scan astrometric measurements (η). For both plots, we removed the best-fit proper motion and overplotted the best-fit planet model (blue). The rest of the figure is a corner plot showing the 2D posterior probability distributions for all model parameters except R.A. and decl. (which were omitted to keep the figure size manageable). Because the distance r and proper motion components μ_α and μ_δ are tightly constrained, we have plotted their offsets from the true values (Δr, Δμ_α, and Δμ_δ). The true values of all parameters (red) were recovered accurately within the uncertainties. This system has SNR₁ ≈ 7; we expect that ∼480 planets in DR4 and ∼2300 in DR5 will have SNR₁ > 7 (see Figure 8).